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Stock award plan - Assumptions Used (Details) - Stock option	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Minimum
Valuation assumptions
Risk-free interest rate	1.84%	1.58%	2.25%
Expected volatility	76.00%	62.00%	64.00%
Expected term	5 years 14 days	5 years 9 months	5 years 10 days
Maximum
Valuation assumptions
Risk-free interest rate	2.45%	2.63%	3.10%
Expected volatility	81.00%	92.00%	90.00%
Expected term	10 years	6 years 1 month 10 days	10 years

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- References

No definition available.

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The estimated measure of the percentage by which a share price is expected to fluctuate during a period. Volatility also may be defined as a probability-weighted measure of the dispersion of returns about the mean. The volatility of a share price is the standard deviation of the continuously compounded rates of return on the share over a specified period. That is the same as the standard deviation of the differences in the natural logarithms of the stock prices plus dividends, if any, over the period.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section 50
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The risk-free interest rate assumption that is used in valuing an option on its own shares.

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Expected term of award under share-based payment arrangement, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents reported fact of one year, five months, and thirteen days.

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